A liquid crystal display (LCD) device is a typical light valve apparatus. These devices display images by modifying the transmission of light. They do not emit light by themselves (liquid crystal panel, for example), but rather block portions of light emitted from a backlight source. To cope with this, light valve display devices are generally provided with a light source (backlight, for example) which emits light from the rear side of a passive light modulation part therein to increase visual brightness of a display screen. Accordingly, brightness of the light source can be adjusted in addition to general contrast adjustment, thereby improving visibility of displayed images.
While the DLP™ technology of Texas Instruments (digital light processing) does not block light in the sense that LCD technology does, the DLP technology uses a large numbers of micro-minors to either reflect light or not reflect light for a duration. The result is in many ways similar to allowing a percentage of light through and blocking the remainder. Liquid crystal on silicon (LCoS) is similar to DLP technology in that it reflects light. LCoS differs from DLP in that it uses liquid crystal technology rather than minors to control the amount of light reflecting off of the device. While the invention disclosed herein applies to a variety of backlit display devices, for clarity and conciseness, the description herein will be primarily limited to discussion of LCD devices.
In recent years, consumer products have used LCD display devices in a variety of industrial applications, displaying information such as data, charts, graphs, and images. These displays are increasingly gaining favor for use in medical imaging devices; including visual field examinations (see U.S. Pat. No. 6,705,726). The LCD display is lightweight, compact, exhibits a multiplicity of colors and is high resolution.
For backlit devices, the backlight luminance generally is factory adjusted to provide the desired brightness of the backlight display device. The backlight luminance can later be controlled by modifications of the backlight drive current or voltage. Normally, backlight luminance is varied dependent upon device temperature and environmental lighting, as well as user preference. For user preference modifications, brightness is typically controlled manually using switches, buttons, or keypads, either directly or remotely to adjust the brightness. In some cases, the backlight display luminance is automatically controlled, based on a light sensor or other environmental sensing device to account for environmental conditions. Since device efficiency may be directly linked to device temperature, one such environmental sensor used by some device manufacturers measures temperature.
Passive light valve devices control the transmission of a background radiation light by adjusting the light transmittance, thereby limiting the total contrast available. Common controllers for adjusting the transmittance are generally 6-bit (64 levels) or 8-bit (256 levels). Controllers exceeding 8-bit are uncommon, largely because of cost-benefit constraints, while controllers with fewer than 64 levels generally do not produce enough levels, though is some cases, the 2-bit controller is used when two colors, say black and white, are sufficient. While gamma curves can change the spacing between adjacent levels, the greatest contrast these devices are capable of is set by the highest and lowest transmittance levels. With only limited levels available, it can be impossible to match the specific intensity of a predefined intensity level or levels, even when the number of levels simultaneously displayed is less than the number of levels controllable by the display.
The industry continues its efforts to improve light valve display devices. Embodiments herein enable backlit display devices including, but not limited to, LCD, LCoS, and DLP devices, to increase the number of levels available from the display device, under certain circumstance, such as when used in visual field examinations.